Solar receiver utilizing carbon nanotube infused coatings

ABSTRACT

A solar receiver includes a heat absorbing element having an outer surface and an inner surface opposite the outer surface and a first coating including a carbon nanotube-infused fiber material in surface engagement with and at least partially covering the outer surface of the heat absorbing element. Solar radiation incident onto the first coating is received, absorbed, and converted to heat energy, and the heat energy is transferred from the first coating to the heat absorbing element. A multilayer coating for a solar receiver device includes a first coating that includes a CNT-infused fiber material and an environmental coating disposed on the first coating.

STATEMENT OF RELATED APPLICATIONS

This application claims priority under 35 U.S.C.119(e) to U.S.Provisional Application 61/167,386 filed Apr. 7, 2009.

FIELD OF INVENTION

The present invention relates in general to a solar receiver apparatusfor receiving, absorbing, containing, and converting receivedelectromagnetic radiation into heat energy.

BACKGROUND

Solar thermal collectors have been developed to harness the energy fromsolar radiation for various industrial processes, power generation andwater heating applications. Solar radiation incident onto the earth'ssurface has an estimated power density of about 1 kW/m² and wavelengthsranging from about 200 nanometers (nm) for ultraviolet (UV) radiation toabout 2500 nm for infrared (IR) radiation. Solar thermal collectorsgenerally include a reflector to focus the solar radiation onto athermal receiver. The thermal receiver converts the photonic energy ofthe solar radiation into thermal energy of a heat transfer fluid.Thermal receivers generally include a thermal absorber which is a goodabsorber of short-wave solar radiation, for example in the UV andvisible range. However, at least some thermal absorbers are also goodlong-wave heat radiators in the infrared range, emitting heat via IRradiation, when sufficiently excited by the absorption of short-wavesolar radiation. Although a high percentage of incident solar radiationmay be initially absorbed, thermal absorbers can emit a high percentageas radiated heat, thereby lowering the effective collection of the solarenergy.

Several types of solar collectors have been developed, including but notlimited to flat plate solar collectors and absorber tubes contained inevacuated glass tube housing. Absorber surfaces can include a bare metalor a metal coated with a selective absorber coating for absorbingradiation within the solar radiation spectrum (i.e., about 200 nm to2500 nm). Such solar selective absorber coatings (having absorptivity,for example, in the range of 0.92 to 0.96 and emissivity, for example,in the range of 0.07 to 0.11) absorb practically all incident radiationbut do not generally emit heat at infra-red wavelengths. Examples ofsuch solar selective absorber coatings include very thin black metallicoxide coating (e.g., on the order of about 0.5 to 1.0 microns) on ahighly reflective metal base, and galvanically applied selectivecoatings such as black chrome, black nickel, and aluminum oxide withnickel. Absorber tubes coated with solar selective coatings aregenerally encased in glass tubes or evacuated glass tubes to minimizethe loss of heat to the ambient air via convection. However, theevacuated glass tubes generally used in conjunction with some of thesecoatings are costly to fabricate and prone to damage when deployed.Additional components such as shrouds are often employed to protect thevacuum seals from direct thermal radiation, which results in losses inefficiency of about 2% Alternative solar receivers having goodabsorbance and low emissivity characteristics are, therefore, desirable.The present invention satisfies this need and provides relatedadvantages as well.

SUMMARY OF THE INVENTION

In some aspects, embodiments disclosed herein relate to a solar receiverthat includes a heat absorbing element having an outer surface and aninner surface opposite the outer surface; and a first coating includinga carbon nanotube-infused fiber material in surface engagement with andat least partially covering the outer surface of the heat absorbingelement. Solar radiation incident onto the first coating is received,absorbed, and converted to heat energy, and the heat energy istransferred from the first coating to the heat absorbing element.

In some aspects, embodiments disclosed herein relate to a multilayercoating for a solar receiver device that includes a first coating havinga CNT-infused fiber material and an environmental coating disposed onthe first coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a profile view of an exemplary solar receiver having aCNT-infused coating on the outer surface of a heat absorbing element.

FIG. 2 is a profile of a solar receiver as shown in FIG. 1, furtherincluding grooves on the outer surface of the heat absorber element.

FIG. 3 is a profile view of a solar receiver as shown in FIG. 1, furtherincluding an environmental coating over the CNT-infused coating.

FIG. 4 is a profile view of a solar receiver as shown in FIG. 3, furtherincluding grooves on the outer surface of the heat absorber element,according to a fourth embodiment of the invention;

FIG. 5 is a cross-sectional view of a ceramic low emissivity,environmental coating integrated into a CNT-infused coating and appliedto an outer surface of a heat absorber element of a solar receiver,according to an embodiment of the invention;

FIG. 6 is a cross-sectional view of the ceramic low emissivity,environmental integrated coating of FIG. 5, further including ananti-reflective coating, according to an embodiment of the invention;

FIG. 7 is a cross-sectional view of a metallic low emissivity,environmental coating applied over the CNT-infused coating, according toan embodiment of the invention;

FIG. 8 is a cross-sectional view of an anti-reflective coating appliedover the metallic low emissivity, environmental integrated coating asshown in FIG. 7, according to an embodiment of the invention;

FIG. 9 is a cross-sectional view of a layered cermet low emissivity,environmental integrated coating applied over the integrated coating asshown in FIG. 5, according to an embodiment of the invention;

FIG. 10 is a cross-sectional view of the layered cermet low emissivity,environmental integrated coating as shown in FIG. 9, further includingan anti-reflective coating, according to an embodiment of the invention;

FIG. 11 is a cross-sectional view of an integrated cermet lowemissivity, environmental CNT-infused coating applied on an outersurface of a heat absorber element of a solar receiver, according to anembodiment of the invention;

FIG. 12 is a cross-sectional view of the integrated cermet lowemissivity, environmental CNT-infused coating as shown in FIG. 11,further including an anti

reflective coating, according to an embodiment of the invention;

FIG. 13 is a cross-sectional view of a solar receiver with an annulus,according to an embodiment of the invention;

FIG. 14 is a cross-sectional view of a solar receiver as shown in FIG.13, further including grooves as described in the second embodimentshown in FIG. 2, according to an embodiment of the invention.

FIG. 15 shows a process for producing CNT-infused carbon fiber materialin accordance with the illustrative embodiment of the present invention.

FIG. 16 shows reflectivity data for a coating that includes aCNT-infused fiber material.

FIG. 17 shows a scanning electron microscope (SEM) image of the CNTsinfused to a fiber material for use in a coating in a solar receiver.

FIG. 18 shows an exemplary solar receiver.

DETAILED DESCRIPTION

The present invention is directed, in part, to a solar receiver thatincorporates a heat absorbing element having a first coating thatincludes a carbon nanotube (CNT)-infused fiber material which serves toabsorb electromagnetic radiation in a wide spectral range fromultraviolet (UV) at about 200 nm through infrared (IR) at about 2500 nm.The CNTs of the CNT-infused fiber material are good thermal conductorsand serve as a conduit for harvesting and converting light energy intoheat. CNTs have some of the highest thermal conductivities known for anymaterial with some indications as high as about 6,600 Wm⁻¹ K⁻¹ (Berberet al. Phys. Rev. Lett. 84(20):4613-4616, (2000)).

Moreover, the fiber material itself of the first coating provides ascaffold to organize the array of infused CNTs with predictablealignments to optimize CNT orientation. CNTs can be fabricated on fibermaterial substrates in controllably aligned configurations in scalablequantities to provide access to large surface area solar receiverpanels. The control of CNT orientation, which is difficult to achievewith “loose” CNT composites, can enhance the light to heat conversion.Control of CNT alignment combined with their high thermal conductivityallows heat to be efficiently and directionally conducted along the CNTlength to the heat absorbing element and from the heating element to aheat transfer fluid for use a variety of applications, including energygeneration.

The solar receivers of the present invention can be used in numerousconventional solar heating collector configurations. For example, thesolar receivers can operate at relatively low temperatures such as thosethat can be used in low-end heating applications such as in a swimmingpool heating system or agricultural uses such as crop drying. The solarreceivers of the present invention can also be used in applications thatemploy high temperatures, including temperatures that are used in energygeneration, such as steam generation, for example. The solar receiversof the present invention can be configured in flat plate designs as wellas parabolic designs.

The coatings employed on solar receivers of the invention can haveabsorptivity, for example, in the range from between about 0.92 to about0.99. Moreover, the emissivity of the solar receiver of the inventioncan be in a range from between about 0.01 to about 0.11. Coatingsemployed in the solar receivers of the invention can absorb almost allincident radiation in a spectral band from the UV through IR, whiletransfer to the heating element and subsequently a heat transfer fluid,prevent thermal infra-red emission. It has been indicated that withproper nanotube density, arrays of vertically aligned single-walled CNTscan behave as nearly perfect black body absorbers (Mizuno et al. Proc.Natl. Acad. Sci. 106:6044-6047 (2009)). One means to generate a blackbody absorber is to suppress light reflection, which can be achievedwhen the refractive index of the object is close to that of air. Thissolution to minimize reflectance is evident from Fresnel's law:

R=(n−n ₀)²/(n+n ₀)²

where R is reflectance, n is the refractive index of the object, and n₀is the refractive index of air. The CNT density on the fiber materialcan be modulated in the continuous process described herein below. Bymodulating CNT density, the CNT-infused fiber material can be tuned toexhibit a refractive index, n, that approximates that of air, n₀.

In some embodiments, the coatings employed in the solar receivers of theinvention having CNT-infused fiber material can behave as ablack-body-like object and can exhibit high thermal emissivity in theform of black body radiation. In some embodiments, this loss of energycan be reduced or prevented by the channeling of the thermal energy fromthe CNTs to the heating absorbing element. The heating absorbingelement, in turn, heats a heat transfer fluid which can be used, forexample, in power generation. Reducing the emissivity of the system canalso be achieved by methods known in the art including, for example,employing vacuum glass chambers about the heating element or employingfurther coating materials, such as anti-reflective coatings or the like.

In some embodiments, the coatings employed in the solar receivers of theinvention having CNT-infused fiber material can behave as intrinsicsolar selective materials that absorb nearly all incident light, whilehave very low emissivity, obviating the need for further coatings,instead efficiently transferring the heat energy to the heat absorbingelement and from the heat absorbing element to the heat transfer fluidfor use in a variety of applications.

In some embodiments, a solar receiver includes a heat absorbing elementhaving an outer surface and an inner surface opposite the outer surface.The receiver further includes a carbon nanotube-infused (“CNT-infused”)material in a first coating in surface engagement with and at leastpartially covering the outer surface of the heat absorbing element.CNT-infused fiber material first coatings include, but are not limitedto, a CNT-infused fiber material and a CNT-infused fiber material in amatrix forming a composite. The solar radiation incident on theCNT-infused fiber material of the first coating is absorbed, contained,and converted to heat energy. The converted heat energy is transferredfrom the CNT-infused fiber material of the first coating on the outersurface of the heat absorbing element to the inner surface of the heatabsorbing element and is then transferred from the inner surface to asubstance such as a heat transfer fluid.

In some embodiments, a solar receiver includes a heat absorbing elementhaving a plurality of grooves on the surface of the heat absorbingelement. In one embodiment, the grooves are on the order of microns (pm)in size and depth. The grooves can be arranged in a spiral configurationalong the circumference of the heat absorbing element to form a singlegroove extending from one end of the heat absorbing element to the otheron the outer surface. Such a groove can accommodate, for example, aCNT-infused fiber tow and can provide enhanced surface contact areabetween the CNT-infused fiber material and the heat absorbing element.Without being bound by theory, this increased surface area contact canprovide more efficient heat transfer to the outer surface of the heatabsorbing element. In a similar manner, an increased surface area can beprovided on the inner surface of the heat absorbing element to increasethe efficiency of heat transfer to the heat transfer fluid.

In some embodiments, a solar receiver includes a low emissivity,environmental coating covering or integrated into the first coatinghaving the CNT-infused fiber material. When integrated into the firstcoating, it can function as a matrix material to provide a first coatingthat is a composite structures. The environmental coating allows for thetransmission of electromagnetic radiation (at least in ultra-violet tovisual range) incident on the outer surface of the environmental coatingonto the CNT-infused fiber material of the first coating for absorptionand conversion to heat energy. The environmental coating has lowemissivity characteristics so as to effectively reduce the emission ofheat energy by the CNT-infused coating back to the external environment.The environmental coating can have a low emissivity, particularly, inthe infra-red spectrum, corresponding to the spectrum at which theCNT-infused fiber material of the first coating emits heat energy at thesystem operating temperature.

In some embodiments, a solar receiver includes an annulus surroundingthe heat absorbing element at least partially covered by the firstcoating having the CNT infused fiber material. In one configuration, theannulus is radially spaced apart from the CNT-infused coating. In anexemplary embodiment, the annulus can include air pockets or air gapsdisposed between the annulus and the CNT-infused coating. In anotherembodiment, the annulus can be evacuated and the gap held under vacuum.The annulus can be coated with one or more of anti-reflective coatingsand low emissivity coatings applied to one or both of its outer andinner surfaces. The annulus can further have infrared reflective coatingapplied to its inner surface which faces the CNT-infused coating.

As used herein the term “fiber material” refers to any material whichhas a fiber as its elementary structural component. The term encompassesfibers, filaments, yarns, tows, tows, tapes, woven and non-wovenfabrics, plies, mats, and the like. Moreover, the composition of thefiber material can be of any type including, without limitation, glass,carbon, metal, ceramic, organic, or the like.

As used herein the term “spoolable dimensions” refers to fiber materialshaving at least one dimension that is not limited in length, allowingfor the material to be stored on a spool or mandrel. Fiber materials of“spoolable dimensions” have at least one dimension that indicates theuse of either batch or continuous processing for CNT infusion asdescribed herein further below. One exemplary fiber material that is acarbon fiber material of spoolable dimensions is commercially availableis exemplified by AS4 12k carbon fiber tow with a tex value of 800 (1tex=1 g/1,000 m) or 620 yard/lb (Grafil, Inc., Sacramento, Calif.).Commercial carbon fiber tow, in particular, can be obtained in 5, 10,20, 50, and 100 lb. (for spools having high weight, usually a 3 k/12Ktow) spools, for example, although larger spools may require specialorder. Processes of the invention operate readily with 5 to 20 lb.spools, although larger spools are usable. Moreover, a pre-processoperation can be incorporated that divides very large spoolable lengths,for example 100 lb. or more, into easy to handle dimensions, such as two50 lb spools.

As used herein, the term “carbon nanotube” (CNT, plural CNTs) refers toany of a number of cylindrically-shaped allotropes of carbon of thefullerene family including single-walled carbon nanotubes (SWNTs),double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes(MWNTs). CNTs can be capped by a fullerene-like structure or open-ended.CNTs include those that encapsulate other materials.

As used herein “uniform in length” refers to length of CNTs grown in areactor. “Uniform length” means that the CNTs have lengths withtolerances of plus or minus about 20% of the total CNT length or less,for CNT lengths varying from between about 1 micron to about 500microns. At very short lengths, such as 1-4 microns, this error may bein a range from between about plus or minus 20% of the total CNT lengthup to about plus or minus 1 micron, that is, somewhat more than about20% of the total CNT length.

As used herein “uniform in distribution” refers to the consistency ofdensity of CNTs on a fiber material. “Uniform distribution” means thatthe CNTs have a density on the fiber material with tolerances of plus orminus about 10% coverage defined as the percentage of the surface areaof the fiber covered by CNTs. This is equivalent to ±1500 CNTs/μm² foran 8 nm diameter CNT with 5 walls. Such a figure assumes the spaceinside the CNTs as fillable.

As used herein, the term “infused” means bonded and “infusion” means theprocess of bonding. Such bonding can involve direct covalent bonding,ionic bonding, pi-pi, and/or van der Waals force-mediated physisorption.For example, in some embodiments, the CNTs can be directly bonded to thefiber material. Bonding can be indirect, such as the CNT infusion to thefiber material via a barrier coating and/or an intervening transitionmetal nanoparticle disposed between the CNTs and fiber material. In theCNT-infused fiber materials disclosed herein, the carbon nanotubes canbe “infused” to the fiber material directly or indirectly as describedabove. The particular manner in which a CNT is “infused” to a carbonfiber materials is referred to as a “bonding motif.”

As used herein, the term “transition metal” refers to any element oralloy of elements in the d-block of the periodic table. The term“transition metal” also includes salt forms of the base transition metalelement such as oxides, carbides, nitrides, and the like.

As used herein, the term “nanoparticle” or NP (plural NPs), orgrammatical equivalents thereof refers to particles sized between about0.1 to about 100 nanometers in equivalent spherical diameter, althoughthe NPs need not be spherical in shape. Transition metal NPs, inparticular, serve as catalysts for CNT growth on the fiber materials.

As used herein, the term “matrix material” refers to a bulk materialthan can serve to organize CNT-infused fiber materials in particularorientations, including random orientation. The matrix material canbenefit from the presence of the CNT-infused carbon fiber material byimparting some aspects of the physical and/or chemical properties of theCNT-infused fiber material to the matrix material. In some embodiments,the matrix material can act as the environmental coating that helpsretain the heat generated upon absorption of solar radiation by theCNTs. In some embodiments, the matrix material is a ceramic. In someembodiments, the matrix material reflects infrared radiation back to theCNTs preventing heat loss to the environment.

As used herein, the term “material residence time” refers to the amountof time a discrete point along a fiber material of spoolable dimensionsis exposed to CNT growth conditions during the CNT infusion processesdescribed herein. This definition includes the residence time whenemploying multiple CNT growth chambers.

As used herein, the term “linespeed” refers to the speed at which afiber material of spoolable dimensions can be fed through the CNTinfusion processes described herein, where linespeed is a velocitydetermined by dividing CNT chamber(s) length by the material residencetime.

In some embodiments, the present invention provides a solar receiverthat includes a heat absorbing element having an outer surface and aninner surface opposite the outer surface; and a first coating thatincludes a carbon nanotube-infused fiber material in surface engagementwith and at least partially covering the outer surface of the heatabsorbing element, whereby solar radiation incident onto the firstcoating is received, absorbed, and converted to heat energy, and theheat energy is transferred from the first coating to said heat absorbingelement.

Solar receivers of the invention can operate low, medium, and hightemperature applications as known in the art. High temperature receiversare used in numerous power generating applications, for example, indriving a turbine with steam. High temperature applications can be anyapplication utilizing temperatures greater than about 400° C. Lowtemperature applications include, for example, pool heating or cropdrying. Such temperatures can be about 10-100° C. higher than ambienttemperatures. Any applications utilizing temperature between about 100°C. and 400° C. are considered mid temperature applications. Exemplarymid temperature application can include, for example, a parabolic troughor concentrating solar power plant.

The solar receiver apparatus has a heat absorbing element having a firstend and a second end and a heat transfer fluid that enters the heatabsorbing element at said first end and exits from the heat absorbingelement at the second end. The heat absorbing element can have grooveson the inner and/or outer surface to provide greater surface areacontact with the first coating on the outside and/or with the heattransfer fluid on the inside of the heat absorbing element. The firstand second ends of the heating element can be used to transport the heattransfer fluid to and from the receiver. The receiver itself isconfigured to integrate into existing systems and can be incorporated inparabolic and flat panel type receivers.

The heat absorbing element is generally a heat pipe made of metal,although any conducting material can be used. Moreover, the heatabsorbing element need not be cylindrical like a pipe. The heatabsorbing element can be any shape and can be chosen for improvedsurface area on the inner and outer surfaces. For example, in someembodiments, the solar receiver heat absorbing element can have groovessized to accommodate the CNT-infused fiber material. When theCNT-infused fiber material is a CNT-infused fiber tow, the grooves canbe helically disposed on the outer surface of the heating element andthe CNT-infused fiber tow wrapped inside the groove and it contact withthe wells of the groove. In some embodiments, when a CNT-infused fibertow is employed, the tow can also be spread onto the heating element.

In some embodiments, the solar receiver of the invention has aCNT-infused fiber material includes a carbon nanotube-infused fiber towthat includes a material selected from carbon, metal, glass, ceramic andthe like.

In some embodiments, the solar receiver of the invention can furtherinclude an environmental coating integrated within said first coating toform a composite. Such materials forming an environmental coatinginclude, without limitation a ceramic matrix material. In someembodiments, the composite formed with the matrix material can furtherinclude metal particles. The metal particles can be used to furtherincrease conductive pathways to disperse the heat collected by the CNTinfused material. They can serve, for example, as conduits for thermalheat transfer between neighboring CNTs, while serving as a infra-redreflector.

In some embodiments, the solar receiver of the invention can furtherinclude an environmental coating disposed on the first coating, and thisenvironmental coating can include a low-emissivity coating. In suchembodiments, the environmental coating can also include the matrix typeenvironmental coating integrated within the CNT-infused fiber material.In some embodiments the environmental coating includes a metal such ascopper.

Solar receivers of the invention can exhibit very low emissivity. Anyenvironmental coating can serve this purpose. Additionally, in someembodiments, the solar receiver of the invention further includes anenvironmental coating that includes an anti-reflective material. Thiscan be used to reflect infrared heat radiated from the CNTs or the heatabsorbing element back towards the CNTs and heating element to preventheat loss to the environment.

In still further embodiments, the solar receiver of the inventionfurther includes an annulus surrounding the first coating and the heatabsorbing element creating a gap. This gap can include air or the gapcan be substantially evacuated.

The solar receivers of the invention are configured to integrate with apower generation system. In this regard, the overall design of thereceiver can be nominally the same as those known in the art.

In some embodiments, the present invention also provides a multilayercoating for a solar receiver device that includes a first coating havinga CNT-infused fiber material; and an environmental coating disposed onthe first coating. The first coating further can include a ceramicmatrix and the first coating can further include metal particles asdescribed above and herein below.

The multilayer coating of the invention can include environmentalcoatings that include a metal film, an anti-reflective coating, and/or alow emissivity coating as described above and further described below.

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in typical solarreceivers and collectors. However, because such elements are well knownin the art, and because they do not facilitate a better understanding ofthe present invention, a discussion of such elements is not providedherein. The disclosure herein is directed to all such variations andmodifications known to those skilled in the art.

Referring to FIG. 1, there is illustrated a profile view of a solarreceiver 100, according to the first embodiment of the invention. Solarreceiver 100 includes a heat absorbing element 110, and a CNT-infusedcoating 120 applied to at least a portion of an outer surface 115 ofheat absorbing element 110.

In one configuration, heat absorbing element 110 is a hollow elementadapted to receive a heat transfer substance, for example, a heattransfer fluid therewithin. By way of non-limiting example only, theheat transfer fluid may include water, anti-freeze solution (e.g., waterand glycol), air, various gases, oil, and other high temperature (highheat capacity) fluids. In an exemplary embodiment, heat absorbingelement 110 is a metallic or alloy absorber tube having a first end 112and a second end 114. Heat absorbing element 110 has an outer surface115 and an inner surface 117 opposite to outer surface 115. By way ofnon-limiting examples only, heat absorbing element 110 may be made ofstainless steel, carbon steel, or aluminum. One skilled in the art willappreciate that other metals and alloys may also be used. The thicknessof heat absorbing element 110 and the material properties of heatabsorbing element 110 are selected to efficiently transfer heat fromouter surface 115 to inner surface 117 which heats a heat transfersubstance present in heat absorbing element 110 generally in surfaceengagement with inner surface 117. In an exemplary configuration, anabsorber tube may have a length of about 3 meters (m), a diameter ofabout 70 millimeters (mm), and a wall thickness of about 2 mm. Whileheat absorber element 110 referred to herein takes the form of a tube ortubular structure, it is understood that heat absorber element 110 maybe configured in various geometric forms, including by way of exampleonly, cylindrical, conical, polygonal or other shapes andconfigurations.

In one configuration, heat absorbing element 110 is an open systemwherein a heat transfer substance such a heat transfer fluid enters atthe first end 112 at a first temperature and exits from the second end114 at a second temperature higher than the first temperature. Inanother configuration, heat absorbing element 110 may be a closedsystem, such as a heat pipe, wherein the heat transfer fluid is retainedwithin heat absorbing element 110. In the illustrated embodiment, heatabsorbing element 110 has an outer surface 115, which is generallyuniform.

Still referring to FIG. 1, CNT-infused coating 120 is disposed on outersurface 115 of heat absorbing element 110. CNT-infused coating 120,therefore, at least partially covers outer surface 115 of heat absorbingelement 110. CNT-infused coating 120 is wound under tension on outersurface 115 of heat absorbing element 110 to establish and maintain aneffective surface engagement or contact with outer surface 115 of heatabsorbing element 110 while minimizing the gaps therebetween. CNT

infused coating 120 receives incident electromagnetic radiation(typically in the form of solar radiation) and converts the receivedradiation into heat or thermal energy. The converted heat or thermalenergy is transferred to outer surface 115 of heat absorber element 110.In an exemplary embodiment, outersurface 115 of heat absorbing element110 is substantially completely covered by CNT-infused coating 120. Inanother embodiment, one or more pre-defined areas of outer surface 115may be left uncovered by CNT-infused coating 120.

In one configuration, CNT-infused coating 120 takes the form of a glassrope or fiber infused with carbon nanotubes. Other examples ofCNT-infused coatings include carbon nanotube-infused fibers and fabrics,such as carbon fibers infused with carbon nanotubes, vapor growth carbonfibers, carbon nanofibers, and graphene. In an exemplary embodiment,CNT-infused coating 120 may have a thickness in the range of about 15microns (pm) to about 1000 pm. CNT-infused coating 120 may optionallyinclude a matrix of a high temperature cement, resin or epoxy, dopedwith carbon nanotubes or metal nanoparticles, to provide structuralintegrity to CNT-infused coating 120.

In an exemplary embodiment, CNT-infused coating 120 may be fabricated inthe form of glass fibers using in situ carbon nanotube growthtechniques. For example, a glass fiber may be fed through a growthchamber maintained at a given temperature of about 5000 to 750° C.Carbon containing feed gas is then introduced into the growth chamber,wherein carbon radicals dissociate and initiate formation of carbonnanotubes on the glass fiber, in presence of catalyst nanoparticles. Onesuch technique is described in the commonly owned Provisional U.S.Application No. 61/155,935, entitled “Low Temperature CNT Growth Using AGas-Pre-heat Method,” and filed Feb. 27, 2009, which application isincorporated by reference herein in its entirety. Other such methods bywhich carbon nanotube infused fibers in the form of a composite coverlayer or thread or rope layer are to be generated may be utilized toobtain CNT-infused coating 120.

As is known in the art, the electromagnetic radiation absorptivity of acarbon nanotube-based structure is, in part, a function of the carbonnanotube length as well as the nanotube volume-filling fraction of thestructure. The nanotube volume-filling fraction represents the fractionof the structure's total volume occupied by the nanotubes. In anexemplary embodiment, the nanotube volume-filling fraction of CNT

infused coating 120 is in the range of about 0.5% to about 25%. Theaverage spacing between the carbon nanotubes in CNT-infused coating 120ranges from about 2 nanometers (nm) to about 200 nm. The nanotube volumefilling of CNT-infused coating 120 may be tailored by selectivepositioning of carbon nanotubes therein to control the range ofelectromagnetic radiation that can be effectively absorbed byCNT-infused coating 120. The gaps between the nanotubes in CNT-infusedcoating 120 may be used to selectively capture and absorb radiationhaving one or more given wavelengths.

The longer the carbon nanotube in the CNT-infused coating, the higherthe absorptivity of electromagnetic radiation (at least in the visiblelight spectrum). CNT

infused coating 120 may include carbon nanotubes having a length in therange of about ten (10) microns to about hundreds of microns.

As is known in the art, thermal conductivity of a carbon nanotube isdependent upon its structural configuration. In particular, the carbonnanotube has a higher thermal conductivity in the direction of itslongitudinal axis as compared with that in a direction perpendicular toits longitudinal axis. In one configuration, CNT-infused coating 120may, therefore, include carbon nanotubes which are aligned generallyperpendicular to outer surface 115, carbon nanotubes which are alignedgenerally parallel to outer surface 115 and carbon nanotubes which arealigned neither parallel nor perpendicular to outer surface 115. Thosecarbon nanotubes generally perpendicular to outer surface 115effectively conduct heat converted from the incident radiation to outersurface 115. Those carbon nanotubes not generally perpendicular to outersurface 115 do not conduct any significant heat to outer surface 115directly. However, those carbon nanotubes not generally perpendicular toouter surface 115, form thermal paths to the generally perpendicularcarbon nanotubes within CNT-infused coating 120, thereby increasingoverall heat transfer from CNT-infused coating 120 to outer surface 115.Thus, the alignment of carbon nanotubes in CNT-infused coating 120 maybe tailored to maximize the thermal conductivity of CNT-infused coating120 to heat absorbing element 110.

Referring now to FIG. 2, there is illustrated a solar receiver 200,according to another embodiment of the invention. Solar receiver 200 isgenerally similar to solar receiver 100. However, receiver 200 has aheat absorbing element 110 having grooves 215 formed on outer surface115. In one configuration, grooves 215 take the form of a spiralconfiguration extending along the length of heat absorbing element 110.It will be appreciated by one skilled in the art that machining a spiralgroove is a simple and well known process. In an exemplary embodiment,grooves 215 may have a size ranging from about 50 pm to about 5000 pm.Grooves 215 effectively increase the surface area of outer surface 115of heat absorbing element 110 exposed to CNT-infused coating 120. Theincreased surface area, in turn, increases the effectiveness of heattransfer from CNT-infused coating 120 to outer surface 115 of heatabsorbing element 110. In an exemplary embodiment, grooves 215 areparticularly effective when combined with a CNT-infused coatingconsisting of CNT-infused fiber tows 120. Groove 215 may be sized tomaximize the contact area between interior surface of groove 215 and theouter surface of one or more individual fibers of CNT-infused coating120. In an exemplary embodiment, groove 215 may be sized to have a sizeand depth approximately similar to a CNT-infused fiber of CNT-infusedcoating 120, thereby accommodating and seating the CNT-infused fiber ofCNT-infused coating 120 in a close fit within groove 215 and maximizingthe surface contact between groove 215 and CNT-infused coating 120. Inother embodiments, groove 215 may accommodate a plurality of CNT-infusedfibers of CNT-infused coating 120.

In one configuration, grooves 215 may take the form of a single groovespirally defined on outer surface 115 and extending continuously alongthe entire length of absorber element 110. In another embodiment,grooves 215 may include a series of discontinuous or segmented groovesdefined on outer surface 115 of heat absorber element 110. Such grooves215 may be aligned longitudinally with one another and sized toaccommodate at least a portion of one or more CNT-infused fibers woundabout absorber element 110.

Referring to FIG. 3, there is illustrated a solar receiver 300,according to another embodiment of the invention. Solar receiver 300 isgenerally similar to solar receiver 100 (of FIG. 1). In oneconfiguration, an environmental coating 310 may be applied to the topsurface of CNT-infused coating 120 to protect CNT-infused coating 120and to improve the reflective and emissive characteristics of thecombination of CNT-infused coating 120 and environmental coating 310.Several embodiments of environmental coating 310 are schematicallydepicted in FIGS. 5-12, and described herein.

Referring now to FIG. 4, there is illustrated a solar receiver 400,according to another embodiment of the invention. Solar receiver 400 isgenerally similar to solar receiver 200 (of FIG. 2), further includingenvironmental coating 310 as described for solar receiver 300 (of FIG.3).

Referring now to FIG. 5, in one configuration of solar receiver 500,there is shown a ceramic environmental coating 510 integrated withCNT-infused coating 120 for protecting CNT-infused coating 120 from theenvironment and for reducing the emission of thermal energy fromCNT-infused coating 120. Environmental coating 510 is transparent to atleast solar radiation to permit the incident radiation to reach CNT

infused coating 120. Furthermore, environmental coating 510 isreflective of thermal radiation, including infra-red radiation, emittedby CNT-infused coating 120, thereby reflecting thermal radiation back toCNT-infused coating 120 for reabsorption. Thus, environmental coating510 has low emissivity characteristics. In an exemplary embodiment,environmental coating 510 may include a ceramic (dielectric) basedmaterial applied as a liquid and converted to a glass through a hightemperature curing cycle. In another embodiment, environmental coating510 may be applied through a chemical vapor deposition process, orthrough plasma sputtering. As such coating application processes areknown in the art, they are not described in further detail for the sakeof brevity. In one configuration, environmental coating 510 is adaptedto withstand high temperatures of CNT-infused coating 120 and heatabsorbing element 110, which may reach as high as 400° to 500° C. Inanother configuration, environmental coating 510 may be adapted to behydrophobic to protect CNT-infused coating 120 from environmentalmoisture. In an exemplary embodiment, environmental coating 510 may havea thickness in the range of about 50 nm to about 500 nm. Examples ofmaterials which may be used to form environmental coating 510 includealumina, silicon dioxide, cesium dioxide, zinc sulfide, aluminumnitride, and zirconium oxide.

Now referring to FIG. 6, in another configuration of solar receiver 600,the integrated ceramic environmental coating 510 and CNT-infused coating120 is further coated with an anti-reflective coating 615. The amount ofincident radiation lost due to reflectance by the integratedenvironmental coating 510 and CNT-infused coating 120 may be reduced bydisposing anti-reflective coating 615 thereon. Anti-reflective coating510, therefore, effectively reduces the reflectance loss of underlyingintegrated environmental coating 510 and CNT-infused coating 120 andincreases the amount of incident radiation absorbed by CNT-infusedcoating 120. Examples of such anti-reflective coatings include magnesiumfluoride, fluoropolymers and silica-based coatings. The use of suchanti-reflective coatings is known in the art and so will not bedescribed in further detail.

Referring to FIG. 7, in one configuration of solar receiver 700, ametallic environmental coating 710 is applied over CNT-infused coating120. In an exemplary embodiment, environmental coating 710 may be ametal thin film that is transparent to at least solar radiation topermit the incident radiation to reach CNT-infused coating 120.Furthermore, environmental coating 710 has low emissivitycharacteristics, by being reflective of thermal radiation, includinginfra-red radiation, from CNT-infused coating 120 back to CNT-infusedcoating 120 for reabsorption. In an exemplary embodiment, environmentalcoating 710 may include a metal thin film material applied through achemical vapor deposition process, or through plasma sputtering orspray. In one configuration, environmental coating 710 is adapted towithstand high temperatures of CNT-infused coating 120 and heatabsorbing element 110, which may reach as high as 400° to 500° C. Inanother configuration, environmental coating 710 may be adapted to behydrophobic. In an exemplary embodiment, environmental coating 710 mayhave a thickness in the range of about 1 nm to about 250 nm. Examples ofmaterials which may be used to form environmental coating 510 include,but not limited to, Molybdenum (Mo), Silver (Ag), Copper (Cu), Nickel(Ni), Titanium (Ti), Platinum (Pt), Tungsten (W), Chromium (Cr), Cobalt(Co), Gold (Au), Cupric oxide (CuO), Cobalt oxide (Co304), Molybdenumdioxide (MoO2), Tungsten oxide (WO), titanium oxide (TiO), Titaniumnitride (TiN), Iron (Fe), and Ferric oxide (Fe203).

Referring now to FIG. 8, in another configuration of solar receiver 800,metallic environmental coating 710 (of FIG. 7) is further coated with ananti-reflective coating 615. Examples of such anti-reflective coatingsinclude magnesium fluoride, fluoropolymers and silica-based coatings.

Referring to FIG. 9, in another configuration of solar receiver 900, theintegrated ceramic environmental coating 510 and CNT-infused coating 120(of FIG. 5) is further coated with a metal coating 710 (of FIG. 7),thereby forming a layered cermet coating on heat absorbing element 110.The layered cermet coating includes metallic coating 710 overlying theintegrated ceramic coating 510 and CNT-infused coating 120. Thecombination of ceramic layer 510 and metallic layer 710 effectivelyincreases the environmental protection provided to CNT-infused coating120 and effectively reduces thermal radiation losses from underlyingCNT-infused coating 120 by reflecting thermal radiations back toCNT-infused coating 120 for reabsorption. The layered cermet layerprovides additional structural integrity to the underlying integratedceramic coating 510 and CNT-infused coating 120.

Referring now to FIG. 10, in another configuration of solar receiver1000, the integrated cermet coatings of FIG. 9 are further coated withan anti-reflective coating 615. Examples of such anti-reflectivecoatings include magnesium fluoride, fluoropolymers and silica-basedcoatings.

Referring now to FIG. 11, in another configuration of solar receiver1100, the integrated ceramic environmental coating 510 and CNT-infusedcoating 120 (of FIG. 5) is doped with metal particles 1110. In oneconfiguration, particles 1110 may include the metals described forcoating 710, and may be applied via colloidal dispersions or selectiveplasma sputtering or sprays. Particle sizes may be between severalmicrons to several nanometers. This configuration thus provides anintegrated layer of CNT

infused coating 120 and integrated ceramic coating 510 doped with metalparticles 1110.

Referring to FIG. 12, in another configuration 1200, the integratedlayer of coatings of FIG. 11 is further coated with an anti-reflectivecoating 615. Examples of such anti-reflective coatings include magnesiumfluoride, fluoropolymers and silica-based coatings.

Referring now to FIG. 13, there is illustrated a solar receiver 1300,according to yet another embodiment of the invention. Solar receiver1300 is generally similar to solar receiver 300 (of FIG. 3). Solarreceiver 1300 additionally includes an annulus 1310 surrounding heatabsorbing element 110 coated with CNT-infused coating 120. In anexemplary embodiment, annulus 1310 takes the form of a glass annulus. Inother embodiments, annulus 1310 may be made of other materials such asquartz or other doped glass materials which are transparent to incidentelectromagnetic radiation, for example, solar radiation. In oneconfiguration, annulus 1310 may be coated with an anti-reflectivecoating on its outer surface, inner surface, or both inner and outersurfaces to maximize the amount of incident radiation transmittedthrough annulus 1310. In an exemplary embodiment, anti-reflectivecoating may include multiple thin film structures having alternatinglayers of contrasting refractive index. Layer thicknesses may be chosento produce destructive interference in the beams reflected from theinterfaces, and constructive interference in the correspondingtransmitted beams. Examples of such anti-reflective coatings includemagnesium fluoride, fluoropolymers and silica-based coatings.

In another configuration, annulus 1310 may be additionally oralternatively coated with a low emissivity coating on the outer, inneror both inner and outer surfaces to reduce radiation heat loss fromemission from annulus 1310. In an exemplary embodiment, a low emissivitycoating is a thin film metal or metallic oxide layer deposited onannulus 1310. Non-limiting examples of such low emissivity coatingsinclude Molybdenum (Mo), Silver (Ag), Copper (Cu), and Nickel (Ni) withthicknesses ranging between 500-50 nm. In yet another configuration,annulus 1310 may be additionally or alternatively coated with aninfra-red reflective coating on its inner, outer, or both inner andouter surfaces. As is known in the art, heat may be from lost throughinfra-red radiation from heat absorbing element 110 covered withCNT-infused coating 120. Annulus 1310 coated with infra-red reflectivecoating reflect such infra-red radiation, emitted by CNT-infused coating120, back to heat absorbing element 110, where CNT-infused coating 120re-absorbs such reflected IR radiation. Thus, effective heat loss frominfra-red radiation is reduced via reabsorption of the emittedradiation. An example of such an infra-red reflective coating is acadmium stannate film.

In an exemplary embodiment, solar receiver 1300 may include air gaps orair pockets between annulus 1310 and heat absorbing element 110 at leastpartially covered with CNT-infused coating 120. In another embodiment,annulus 1310 may be evacuated to reduce heat loss due to convection inthe air present between CNT

infused coating 120 and annulus 1310. In yet another exemplaryembodiment, solar receiver 1300 may further include one or more of theenvironmental, low emissivity coatings described in relation to FIGS.5-12.

Referring now to FIG. 14, a solar receiver 1400 is illustrated accordingto an embodiment of the invention. Solar receiver 1400 is generallysimilar to solar receiver 400. Solar receiver 400 additionally includesan annulus 1310 surrounding heat absorbing element 110 at leastpartially covered with CNT-infused coating 120. Annulus 1310 may becoated with one or more of anti-reflective coating on its outer, inneror both outer and inner surfaces, low emissivity coating on its outer,inner, or inner and outer surfaces, infra-red radiation reflectivecoating on its inner, outer, or inner and outer surfaces, as describedabove herein with regard to the embodiments of FIG. 13. In yet anotherexemplary embodiment, solar receiver 1400 may further include one ormore of the environmental, low emissivity coatings described in relationto FIGS. 5-12.

Below is an exemplary process for generating a CNT infused fibermaterial. This process is exemplified with carbon fiber material,however, one skilled in the art will appreciate that the operationalparameters will be similar for other material types, including glass,ceramic, and metal fiber materials as well.

In some embodiments the present invention provides a continuous processfor CNT infusion that includes (a) disposing a carbon nanotube-formingcatalyst on a surface of a fiber material of spoolable dimensions; and(b) synthesizing carbon nanotubes directly on the fiber material,thereby forming a carbon nanotube-infused fiber material. For a 9 footlong system, the linespeed of the process can range from between about1.5 ft/min to about 108 ft/min. The linespeeds achieved by the processdescribed herein allow the formation of commercially relevant quantitiesof CNT-infused fiber materials with short production times. For example,at 36 ft/min linespeed, the quantities of CNT-infused fibers (over 5%infused CNTs on fiber by weight) can exceed over 100 pound or more ofmaterial produced per day in a system that is designed to simultaneouslyprocess 5 separate tows (20 lb/tow). Systems can be made to produce moretows at once or at faster speeds by repeating growth zones. Moreover,some steps in the fabrication of CNTs, as known in the art, haveprohibitively slow rates preventing a continuous mode of operation. Forexample, in a typical process known in the art, a CNT-forming catalystreduction step can take 1-12 hours to perform. CNT growth itself canalso be time consuming, for example requiring tens of minutes for CNTgrowth, precluding the rapid linespeeds realized in the presentinvention. The process described herein overcomes such rate limitingsteps.

The CNT-infused fiber material-forming processes of the invention canavoid CNT bundling that occurs when trying to apply suspensions ofpre-formed carbon nanotubes to fiber materials. That is, becausepre-formed CNTs are not fused to the carbon fiber material, the CNTstend to bundle and entangle. The result is a poorly uniform distributionof CNTs that weakly adhere to the carbon fiber material. However,processes of the present invention can provide, if desired, a highlyuniform entangled CNT mat on the surface of the fiber material byreducing the growth density. The CNTs grown at low density are infusedin the fiber material first. In such embodiments, the fibers do not growdense enough to induce vertical alignment, the result is entangled matson the carbon fiber material surfaces. By contrast, manual applicationof pre-formed CNTs does not insure uniform distribution and density of aCNT mat on the carbon fiber material.

FIG. 15 depicts a flow diagram of process 1500 for producing CNT-infusedcarbon fiber material in accordance with an illustrative embodiment ofthe present invention. Again, the use of a carbon fiber material ismerely exemplary.

Process 1500 includes at least the operations of:

1501: Functionalizing the carbon fiber material.

1502: Applying a barrier coating and a CNT-forming catalyst to thefunctionalized carbon fiber material.

1504: Heating the carbon fiber material to a temperature that issufficient for carbon nanotube synthesis.

1506: Promoting CVD-mediated CNT growth on the catalyst-laden carbonfiber.

In step 1501, the carbon fiber material is functionalized to promotesurface wetting of the fibers and to improve adhesion of the barriercoating.

To infuse carbon nanotubes into a carbon fiber material, the carbonnanotubes are synthesized on the carbon fiber material which isconformally coated with a barrier coating.

In one embodiment, this is accomplished by first conformally coating thecarbon fiber material with a barrier coating and then disposingnanotube-forming catalyst on the barrier coating, as per operation 1502.In some embodiments, the barrier coating can be partially cured prior tocatalyst deposition. This can provide a surface that is receptive toreceiving the catalyst and allowing it to embed in the barrier coating,including allowing surface contact between the CNT forming catalyst andthe carbon fiber material. In such embodiments, the barrier coating canbe fully cured after embedding the catalyst. In some embodiments, thebarrier coating is conformally coated over the carbon fiber materialsimultaneously with deposition of the CNT-form catalyst. Once theCNT-forming catalyst and barrier coating are in place, the barriercoating can be fully cured.

In some embodiments, the barrier coating can be fully cured prior tocatalyst deposition. In such embodiments, a fully cured barrier-coatedcarbon fiber material can be treated with a plasma to prepare thesurface to accept the catalyst. For example, a plasma treated carbonfiber material having a cured barrier coating can provide a roughenedsurface in which the CNT-forming catalyst can be deposited. The plasmaprocess for “roughing” the surface of the barrier thus facilitatescatalyst deposition. The roughness is typically on the scale ofnanometers. In the plasma treatment process craters or depressions areformed that are nanometers deep and nanometers in diameter. Such surfacemodification can be achieved using a plasma of any one or more of avariety of different gases, including, without limitation, argon,helium, oxygen, nitrogen, and hydrogen. In some embodiments, plasmaroughing can also be performed directly in the carbon fiber materialitself. This can facilitate adhesion of the barrier coating to thecarbon fiber material.

As described further below and in conjunction with FIG. 15, the catalystis prepared as a liquid solution that contains CNT-forming catalyst thatcomprise transition metal nanoparticles. The diameters of thesynthesized nanotubes are related to the size of the metal particles asdescribed above. In some embodiments, commercial dispersions ofCNT-forming transition metal nanoparticle catalyst are available and areused without dilution, in other embodiments commercial dispersions ofcatalyst can be diluted. Whether to dilute such solutions can depend onthe desired density and length of CNT to be grown as described above.

With reference to the illustrative embodiment of FIG. 15, carbonnanotube synthesis is shown based on a chemical vapor deposition (CVD)process and occurs at elevated temperatures. The specific temperature isa function of catalyst choice, but will typically be in a range of about500 to 1000° C. Accordingly, operation 1504 involves heating thebarrier-coated carbon fiber material to a temperature in theaforementioned range to support carbon nanotube synthesis.

In operation 1506, CVD-promoted nanotube growth on the catalyst-ladencarbon fiber material is then performed. The CVD process can be promotedby, for example, a carbon-containing feedstock gas such as acetylene,ethylene, and/or ethanol. The CNT synthesis processes generally use aninert gas (nitrogen, argon, helium) as a primary carrier gas. The carbonfeedstock is provided in a range from between about 0% to about 15% ofthe total mixture. A substantially inert environment for CVD growth isprepared by removal of moisture and oxygen from the growth chamber.

In the CNT synthesis process, CNTs grow at the sites of a CNT-formingtransition metal nanoparticle catalyst. The presence of the strongplasma-creating electric field can be optionally employed to affectnanotube growth. That is, the growth tends to follow the direction ofthe electric field. By properly adjusting the geometry of the plasmaspray and electric field, vertically-aligned CNTs (i.e., perpendicularto the carbon fiber material) can be synthesized. Under certainconditions, even in the absence of a plasma, closely-spaced nanotubeswill maintain a vertical growth direction resulting in a dense array ofCNTs resembling a carpet or forest. The presence of the barrier coatingcan also influence the directionality of CNT growth.

The operation of disposing a catalyst on the carbon fiber material canbe accomplished by spraying or dip coating a solution or by gas phasedeposition via, for example, a plasma process. The choice of techniquescan be coordinated with the mode with which the barrier coating isapplied. Thus, in some embodiments, after forming a solution of acatalyst in a solvent, catalyst can be applied by spraying or dipcoating the barrier coated carbon fiber material with the solution, orcombinations of spraying and dip coating. Either technique, used aloneor in combination, can be employed once, twice, thrice, four times, upto any number of times to provide a carbon fiber material that issufficiently uniformly coated with CNT-forming catalyst. When dipcoating is employed, for example, a carbon fiber material can be placedin a first dip bath for a first residence time in the first dip bath.When employing a second dip bath, the carbon fiber material can beplaced in the second dip bath for a second residence time. For example,carbon fiber materials can be subjected to a solution of CNT-formingcatalyst for between about 3 seconds to about 90 seconds depending onthe dip configuration and linespeed. Employing spraying or dip coatingprocesses, a carbon fiber material with a surface density of catalyst ofless than about 5% surface coverage to as high as about 80% coverage, inwhich the CNT-forming catalyst nanoparticles are nearly monolayer. Insome embodiments, the process of coating the CNT-forming catalyst on thecarbon fiber material should produce no more than a monolayer. Forexample, CNT growth on a stack of CNT-forming catalyst can erode thedegree of infusion of the CNT to the carbon fiber material. In otherembodiments, the transition metal catalyst can be deposited on thecarbon fiber material using evaporation techniques, electrolyticdeposition techniques, and other processes known to those skilled in theart, such as addition of the transition metal catalyst to a plasmafeedstock gas as a metal organic, metal salt or other compositionpromoting gas phase transport.

Because processes of the invention are designed to be continuous, aspoolable carbon fiber material can be dip-coated in a series of bathswhere dip coating baths are spatially separated. In a continuous processin which nascent carbon fibers are being generated de novo, dip bath orspraying of CNT-forming catalyst can be the first step after applyingand curing or partially curing a barrier coating to the carbon fibermaterial. Application of the barrier coating and a CNT-forming catalystcan be performed in lieu of application of a sizing, for newly formedcarbon fiber materials. In other embodiments, the CNT-forming catalystcan be applied to newly formed carbon fibers in the presence of othersizing agents after barrier coating. Such simultaneous application ofCNT-forming catalyst and other sizing agents can still provide theCNT-forming catalyst in surface contact with the barrier coating of thecarbon fiber material to insure CNT infusion.

The catalyst solution employed can be a transition metal nanoparticlewhich can be any d-block transition metal as described above. Inaddition, the nanoparticles can include alloys and non-alloy mixtures ofd-block metals in elemental form or in salt form, and mixtures thereof.Such salt forms include, without limitation, oxides, carbides, andnitrides. Non-limiting exemplary transition metal NPs include Ni, Fe,Co, Mo, Cu, Pt, Au, and Ag and salts thereof and mixtures thereof. Insome embodiments, such CNT-forming catalysts are disposed on the carbonfiber by applying or infusing a CNT-forming catalyst directly to thecarbon fiber material simultaneously with barrier coating deposition.Many of these transition metal catalysts are readily commerciallyavailable from a variety of suppliers, including, for example, FerrotecCorporation (Bedford, N.H.).

Catalyst solutions used for applying the CNT-forming catalyst to thecarbon fiber material can be in any common solvent that allows theCNT-forming catalyst to be uniformly dispersed throughout. Such solventscan include, without limitation, water, acetone, hexane, isopropylalcohol, toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexaneor any other solvent with controlled polarity to create an appropriatedispersion of the CNT-forming catalyst nanoparticles. Concentrations ofCNT-forming catalyst can be in a range from about 1:1 to 1:10000catalyst to solvent. Such concentrations can be used when the barriercoating and CNT-forming catalyst is applied simultaneously as well.

In some embodiments heating of the carbon fiber material can be at atemperature that is between about 500° C. and 1000° C. to synthesizecarbon nanotubes after deposition of the CNT-forming catalyst. Heatingat these temperatures can be performed prior to or substantiallysimultaneously with introduction of a carbon feedstock for CNT growth.

In some embodiments, the present invention provides a process thatincludes removing sizing agents from a carbon fiber material, applying abarrier coating conformally over the carbon fiber material, applying aCNT-forming catalyst to the carbon fiber material, heating the carbonfiber material to at least 500° C., and synthesizing carbon nanotubes onthe carbon fiber material. In some embodiments, operations of theCNT-infusion process include removing sizing from a carbon fibermaterial, applying a barrier coating to the carbon fiber material,applying a CNT-forming catalyst to the carbon fiber, heating the fiberto CNT-synthesis temperature and CVD-promoted CNT growth thecatalyst-laden carbon fiber material. Thus, where commercial carbonfiber materials are employed, processes for constructing CNT-infusedcarbon fibers can include a discrete step of removing sizing from thecarbon fiber material before disposing barrier coating and the catalyston the carbon fiber material.

The step of synthesizing carbon nanotubes can include numeroustechniques for forming carbon nanotubes, including those disclosed inco-pending U.S. Patent Application No. US 2004/0245088 which isincorporated herein by reference. The CNTs grown on fibers of thepresent invention can be accomplished by techniques known in the artincluding, without limitation, micro-cavity, thermal or plasma-enhancedCVD techniques, laser ablation, arc discharge, and high pressure carbonmonoxide (HiPCO). During CVD, in particular, a barrier coated carbonfiber material with CNT-forming catalyst disposed thereon, can be useddirectly. In some embodiments, any conventional sizing agents can beremoved prior CNT synthesis. In some embodiments, acetylene gas isionized to create a jet of cold carbon plasma for CNT synthesis. Theplasma is directed toward the catalyst-bearing carbon fiber material.Thus, in some embodiments synthesizing CNTs on a carbon fiber materialincludes (a) forming a carbon plasma; and (b) directing the carbonplasma onto the catalyst disposed on the carbon fiber material. Thediameters of the CNTs that are grown are dictated by the size of theCNT-forming catalyst as described above. In some embodiments, the sizedfiber substrate is heated to between about 550 to about 800° C. tofacilitate CNT synthesis. To initiate the growth of CNTs, two gases arebled into the reactor: a process gas such as argon, helium, or nitrogen,and a carbon-containing gas, such as acetylene, ethylene, ethanol ormethane. CNTs grow at the sites of the CNT-forming catalyst.

In some embodiments, the CVD growth is plasma-enhanced. A plasma can begenerated by providing an electric field during the growth process. CNTsgrown under these conditions can follow the direction of the electricfield. Thus, by adjusting the geometry of the reactor vertically alignedcarbon nanotubes can be grown radially about a cylindrical fiber. Insome embodiments, a plasma is not required for radial growth about thefiber. For carbon fiber materials that have distinct sides such astapes, mats, fabrics, plies, and the like, catalyst can be disposed onone or both sides and correspondingly, CNTs can be grown on one or bothsides as well.

As described above, CNT-synthesis is performed at a rate sufficient toprovide a continuous process for functionalizing spoolable carbon fibermaterials. Numerous apparatus configurations faciliate such continuoussynthesis as exemplified below.

In some embodiments, CNT-infused carbon fiber materials can beconstructed in an “all plasma” process. An all plasma process can beingwith roughing the carbon fiber material with a plasma as described aboveto improve fiber surface wetting characteristics and provide a moreconformal barrier coating, as well as improve coating adhesion viamechanical interlocking and chemical adhesion through the use offunctionalization of the carbon fiber material by using specificreactive gas species, such as oxygen, nitrogen, hydrogen in argon orhelium based plasmas.

Barrier coated carbon fiber materials pass through numerous furtherplasma-mediated steps to form the final CNT-infused product. In someembodiments, the all plasma process can include a second surfacemodification after the barrier coating is cured. This is a plasmaprocess for “roughing” the surface of the barrier coating on the carbonfiber material to facilitate catalyst deposition. As described above,surface modification can be achieved using a plasma of any one or moreof a variety of different gases, including, without limitation, argon,helium, oxygen, ammonia, hydrogen, and nitrogen.

After surface modification, the barrier coated carbon fiber materialproceeds to catalyst application. This is a plasma process fordepositing the CNT-forming catalyst on the fibers. The CNT-formingcatalyst is typically a transition metal as described above. Thetransition metal catalyst can be added to a plasma feedstock gas as aprecursor in the form of a ferrofluid, a metal organic, metal salt orother composition for promoting gas phase transport. The catalyst can beapplied at room temperature in the ambient environment with neithervacuum nor an inert atmosphere being required. In some embodiments, thecarbon fiber material is cooled prior to catalyst application.

Continuing the all-plasma process, carbon nanotube synthesis occurs in aCNT-growth reactor. This can be achieved through the use ofplasma-enhanced chemical vapor deposition, wherein carbon plasma issprayed onto the catalyst-laden fibers. Since carbon nanotube growthoccurs at elevated temperatures (typically in a range of about 500 to1000° C. depending on the catalyst), the catalyst-laden fibers can beheated prior to exposing to the carbon plasma. For the infusion process,the carbon fiber material can be optionally heated until it softens.After heating, the carbon fiber material is ready to receive the carbonplasma. The carbon plasma is generated, for example, by passing a carboncontaining gas such as acetylene, ethylene, ethanol, and the like,through an electric field that is capable of ionizing the gas. This coldcarbon plasma is directed, via spray nozzles, to the carbon fibermaterial. The carbon fiber material can be in close proximity to thespray nozzles, such as within about 1 centimeter of the spray nozzles,to receive the plasma. In some embodiments, heaters are disposed abovethe carbon fiber material at the plasma sprayers to maintain theelevated temperature of the carbon fiber material.

Another configuration for continuous carbon nanotube synthesis involvesa special rectangular reactor for the synthesis and growth of carbonnanotubes directly on carbon fiber materials. The reactor can bedesigned for use in a continuous in-line process for producingcarbon-nanotube bearing fibers. In some embodiments, CNTs are grown viaa chemical vapor deposition (“CVD”) process at atmospheric pressure andat elevated temperature in the range of about 550° C. to about 800° C.in a multi-zone reactor. The fact that the synthesis occurs atatmospheric pressure is one factor that facilitates the incorporation ofthe reactor into a continuous processing line for CNT-on-fibersynthesis. Another advantage consistent with in-line continuousprocessing using such a zone reactor is that CNT growth occurs in aseconds, as opposed to minutes (or longer) as in other procedures andapparatus configurations typical in the art.

CNT synthesis reactors in accordance with the various embodimentsinclude the following features:

Rectangular Configured Synthesis Reactors: The cross section of atypical CNT synthesis reactor known in the art is circular. There are anumber of reasons for this including, for example, historical reasons(cylindrical reactors are often used in laboratories) and convenience(flow dynamics are easy to model in cylindrical reactors, heater systemsreadily accept circular tubes (quartz, etc.), and ease of manufacturing.Departing from the cylindrical convention, the present inventionprovides a CNT synthesis reactor having a rectangular cross section. Thereasons for the departure are as follows: 1. Since many carbon fibermaterials that can be processed by the reactor are relatively planarsuch as flat tape or sheet-like in form, a circular cross section is aninefficient use of the reactor volume. This inefficiency results inseveral drawbacks for cylindrical CNT synthesis reactors including, forexample, a) maintaining a sufficient system purge; increased reactorvolume requires increased gas flow rates to maintain the same level ofgas purge. This results in a system that is inefficient for high volumeproduction of CNTs in an open environment; b) increased carbon feedstockgas flow; the relative increase in inert gas flow, as per a) above,requires increased carbon feedstock gas flows. Consider that the volumeof a 12K carbon fiber tow is 2000 times less than the total volume of asynthesis reactor having a rectangular cross section. In an equivalentgrowth cylindrical reactor (i.e., a cylindrical reactor that has a widththat accommodates the same planarized carbon fiber material as therectangular cross-section reactor), the volume of the carbon fibermaterial is 17,500 times less than the volume of the chamber. Althoughgas deposition processes, such as CVD, are typically governed bypressure and temperature alone, volume has a significant impact on theefficiency of deposition. With a rectangular reactor there is a stillexcess volume. This excess volume facilitates unwanted reactions; yet acylindrical reactor has about eight times that volume. Due to thisgreater opportunity for competing reactions to occur, the desiredreactions effectively occur more slowly in a cylindrical reactorchamber. Such a slow down in CNT growth, is problematic for thedevelopment of a continuous process. One benefit of a rectangularreactor configuration is that the reactor volume can be decreased byusing a small height for the rectangular chamber to make this volumeratio better and reactions more efficient. In some embodiments of thepresent invention, the total volume of a rectangular synthesis reactoris no more than about 3000 times greater than the total volume of acarbon fiber material being passed through the synthesis reactor. Insome further embodiments, the total volume of the rectangular synthesisreactor is no more than about 4000 times greater than the total volumeof the carbon fiber material being passed through the synthesis reactor.In some still further embodiments, the total volume of the rectangularsynthesis reactor is less than about 10,000 times greater than the totalvolume of the carbon fiber material being passed through the synthesisreactor. Additionally, it is notable that when using a cylindricalreactor, more carbon feedstock gas is required to provide the same flowpercent as compared to reactors having a rectangular cross section. Itshould be appreciated that in some other embodiments, the synthesisreactor has a cross section that is described by polygonal forms thatare not rectangular, but are relatively similar thereto and provide asimilar reduction in reactor volume relative to a reactor having acircular cross section; c) problematic temperature distribution; when arelatively small-diameter reactor is used, the temperature gradient fromthe center of the chamber to the walls thereof is minimal. But withincreased size, such as would be used for commercial-scale production,the temperature gradient increases. Such temperature gradients result inproduct quality variations across a carbon fiber material substrate(i.e., product quality varies as a function of radial position). Thisproblem is substantially avoided when using a reactor having arectangular cross section. In particular, when a planar substrate isused, reactor height can be maintained constant as the size of thesubstrate scales upward. Temperature gradients between the top andbottom of the reactor are essentially negligible and, as a consequence,thermal issues and the product-quality variations that result areavoided. 2. Gas introduction: Because tubular furnaces are normallyemployed in the art, typical CNT synthesis reactors introduce gas at oneend and draw it through the reactor to the other end. In someembodiments disclosed herein, gas can be introduced at the center of thereactor or within a target growth zone, symmetrically, either throughthe sides or through the top and bottom plates of the reactor. Thisimproves the overall CNT growth rate because the incoming feedstock gasis continuously replenishing at the hottest portion of the system, whichis where CNT growth is most active. This constant gas replenishment isan important aspect to the increased growth rate exhibited by therectangular CNT reactors.

Zoning. Chambers that provide a relatively cool purge zone depend fromboth ends of the rectangular synthesis reactor. Applicants havedetermined that if hot gas were to mix with the external environment(i.e., outside of the reactor), there would be an increase indegradation of the carbon fiber material. The cool purge zones provide abuffer between the internal system and external environments. TypicalCNT synthesis reactor configurations known in the art typically requirethat the substrate is carefully (and slowly) cooled. The cool purge zoneat the exit of the present rectangular CNT growth reactor achieves thecooling in a short period of time, as required for the continuousin-line processing.

Non-contact, hot-walled, metallic reactor. In some embodiments, ahot-walled reactor is made of metal is employed, in particular stainlesssteel. This may appear counterintuitive because metal, and stainlesssteel in particular, is more susceptible to carbon deposition (i.e.,soot and by-product formation). Thus, most CNT reactor configurationsuse quartz reactors because there is less carbon deposited, quartz iseasier to clean, and quartz facilitates sample observation. However,Applicants have observed that the increased soot and carbon depositionon stainless steel results in more consistent, faster, more efficient,and more stable CNT growth. Without being bound by theory it has beenindicated that, in conjunction with atmospheric operation, the CVDprocess occurring in the reactor is diffusion limited. That is, thecatalyst is “overfed;” too much carbon is available in the reactorsystem due to its relatively higher partial pressure (than if thereactor was operating under partial vacuum). As a consequence, in anopen system—especially a clean one—too much carbon can adhere tocatalyst particles, compromising their ability to synthesize CNTs. Insome embodiments, the rectangular reactor is intentionally run when thereactor is “dirty,” that is with soot deposited on the metallic reactorwalls. Once carbon deposits to a monolayer on the walls of the reactor,carbon will readily deposit over itself. Since some of the availablecarbon is “withdrawn” due to this mechanism, the remaining carbonfeedstock, in the form of radicals, react with the catalyst at a ratethat does not poison the catalyst. Existing systems run “cleanly” which,if they were open for continuous processing, would produced a much loweryield of CNTs at reduced growth rates.

Although it is generally beneficial to perform CNT synthesis “dirty” asdescribed above, certain portions of the apparatus, such as gasmanifolds and inlets, can nonetheless negatively impact the CNT growthprocess when soot created blockages. In order to combat this problem,such areas of the CNT growth reaction chamber can be protected with sootinhibiting coatings such as silica, alumina, or MgO. In practice, theseportions of the apparatus can be dip-coated in these soot inhibitingcoatings. Metals such as INVAR® can be used with these coatings as INVARhas a similar CTE (coefficient of thermal expansion) ensuring properadhesion of the coating at higher temperatures, preventing the soot fromsignificantly building up in critical zones.

Combined Catalyst Reduction and CNT Synthesis. In the CNT synthesisreactor disclosed herein, both catalyst reduction and CNT growth occurwithin the reactor. This is significant because the reduction stepcannot be accomplished timely enough for use in a continuous process ifperformed as a discrete operation. In a typical process known in theart, a reduction step typically takes 1-12 hours to perform. Bothoperations occur in a reactor in accordance with the present inventiondue, at least in part, to the fact that carbon feedstock gas isintroduced at the center of the reactor, not the end as would be typicalin the art using cylindrical reactors. The reduction process occurs asthe fibers enter the heated zone; by this point, the gas has had time toreact with the walls and cool off prior to reacting with the catalystand causing the oxidation reduction (via hydrogen radical interactions).It is this transition region where the reduction occurs. At the hottestisothermal zone in the system, the CNT growth occurs, with the greatestgrowth rate occurring proximal to the gas inlets near the center of thereactor.

In some embodiments, when loosely affiliated carbon fiber materials,such as carbon tow are employed, the continuous process can includesteps that spreads out the strands and/or filaments of the tow. Thus, asa tow is unspooled it can be spread using a vacuum-based fiber spreadingsystem, for example. When employing sized carbon fibers, which can berelatively stiff, additional heating can be employed in order to“soften” the tow to facilitate fiber spreading. The spread fibers whichcomprise individual filaments can be spread apart sufficiently to exposean entire surface area of the filaments, thus allowing the tow to moreefficiently react in subsequent process steps. Such spreading canapproach between about 4 inches to about 6 inches across for a 3 k tow.The spread carbon tow can pass through a surface treatment step that iscomposed of a plasma system as described above. After a barrier coatingis applied and roughened, spread fibers then can pass through aCNT-forming catalyst dip bath. The result is fibers of the carbon towthat have catalyst particles distributed radially on their surface. Thecatalyzed-laden fibers of the tow then enter an appropriate CNT growthchamber, such as the rectangular chamber described above, where a flowthrough atmospheric pressure CVD or PE-CVD process is used to synthesizethe CNTs at rates as high as several microns per second. The fibers ofthe tow, now with radially aligned CNTs, exit the CNT growth reactor.

In some embodiments, CNT-infused carbon fiber materials can pass throughyet another treatment process that, in some embodiments is a plasmaprocess used to functionalize the CNTs. Additional functionalization ofCNTs can be used to promote their adhesion to particular resins. Thus,in some embodiments, the present invention provides CNT-infused carbonfiber materials having functionalized CNTs.

As part of the continuous processing of spoolable carbon fibermaterials, the a CNT-infused carbon fiber material can further passthrough a sizing dip bath to apply any additional sizing agents whichcan be beneficial in a final product. Finally if wet winding is desired,the CNT-infused carbon fiber materials can be passed through a resinbath and wound on a mandrel or spool. The resulting carbon fibermaterial/resin combination locks the CNTs on the carbon fiber materialallowing for easier handling and composite fabrication. In someembodiments, CNT infusion is used to provide improved filament winding.Thus, CNTs formed on carbon fibers such as carbon tow, are passedthrough a resin bath to produce resin-impregnated, CNT-infused carbontow. After resin impregnation, the carbon tow can be positioned on thesurface of a rotating mandrel by a delivery head. The tow can then bewound onto the mandrel in a precise geometric pattern in known fashion.

The winding process described above provides pipes, tubes, or otherforms as are characteristically produced via a male mold. But the formsmade from the winding process disclosed herein differ from thoseproduced via conventional filament winding processes. Specifically, inthe process disclosed herein, the forms are made from compositematerials that include CNT-infused tow. Such forms will thereforebenefit from enhanced strength and the like, as provided by theCNT-infused tow.

In some embodiments, a continuous process for infusion of CNTs onspoolable carbon fiber materials can achieve a linespeed between about0.5 ft/min to about 36 ft/min. In this embodiment where the CNT growthchamber is 3 feet long and operating at a 750° C. growth temperature,the process can be run with a linespeed of about 6 ft/min to about 36ft/min to produce, for example, CNTs having a length between about 1micron to about 10 microns. The process can also be run with a linespeedof about 1 ft/min to about 6 ft/min to produce, for example, CNTs havinga length between about 10 microns to about 100 microns. The process canbe run with a linespeed of about 0.5 ft/min to about 1 ft/min toproduce, for example, CNTs having a length between about 100 microns toabout 200 microns. The CNT length is not tied only to linespeed andgrowth temperature, however, the flow rate of both the carbon feedstockand the inert carrier gases can also influence CNT length. For example,a flow rate consisting of less than 1% carbon feedstock in inert gas athigh linespeeds (6 ft/min to 36 ft/min) will result in CNTs having alength between 1 micron to about 5 microns. A flow rate consisting ofmore than 1% carbon feedstock in inert gas at high linespeeds (6 ft/minto 36 ft/min) will result in CNTs having length between 5 microns toabout 10 microns.

In some embodiments, more than one carbon material can be runsimultaneously through the process. For example, multiple tapes tows,filaments, strand and the like can be run through the process inparallel. Thus, any number of pre-fabricated spools of carbon fibermaterial can be run in parallel through the process and re-spooled atthe end of the process. The number of spooled carbon fiber materialsthat can be run in parallel can include one, two, three, four, five,six, up to any number that can be accommodated by the width of theCNT-growth reaction chamber. Moreover, when multiple carbon fibermaterials are run through the process, the number of collection spoolscan be less than the number of spools at the start of the process. Insuch embodiments, carbon strands, tows, or the like can be sent througha further process of combining such carbon fiber materials into higherordered carbon fiber materials such as woven fabrics or the like. Thecontinuous process can also incorporate a post processing chopper thatfacilitates the formation CNT-infused chopped fiber mats, for example.

It is understood that modifications which do not substantially affectthe activity of the various embodiments of this invention are alsoincluded within the definition of the invention provided herein.Accordingly, the following example is intended to illustrate but notlimit the present invention.

Example I

This example shows the manufacture of CNT infused coating for use in asolar receiver and characterization of a model.

A CNT based coating can be manufactured by the following procedure:

CNTs are infused to a carbon fiber tow (carbon fiber being exemplary) ina reel-to-reel system as outlined above. The CNT infused fiber tow isthen wrapped over a heating element. Additional reflective layers areadded as needed. A coating made by this procedure is expected to exhibitcharacteristics of being a solar selective coating. The exactcharacteristics of a coating employing CNT-infused fibers will depend onCNT length and density.

FIG. 16 shows the reflectivity data for a model of this CNT-infusedfiber coating, namely Buckypaper, with an overlay of a theoretical idealcoating indicated as a dashed line. The CNT-infused fiber wrapped arounda heating element has an arrangement of CNTs similar to Buckypaper. Thearrangement of CNTs in Buckypaper are shown in the SEM image of FIG. 17.

The coating having CNT-infused fiber can be formed onto to the outersurface of a heat absorber element for incorporation into a solarreceiver, such as the one exemplified in FIG. 18. This solar receiverincludes an annulus surrounding the heat absorbing element coated withCNT-infused coating. The annulus can be borosilicate glass with ananti-reflective coating on its outer surface, inner surface, or bothinner and outer surfaces to maximize the amount of incident radiationtransmitted through annulus. The annulus can be evacuated to a pressure(less than or equal to 0.0001 Torr) to minimize heat loss due toconvection in the air present between CNT-infused coating and annulus.

While the foregoing invention has been described with reference to theabove-described embodiment, various modifications and changes can bemade without departing from the spirit of the invention. Accordingly,all such modifications and changes are considered to be within the scopeof the appended claims.

1. A solar receiver comprising: a heat absorbing element having an outersurface and an inner surface opposite the outer surface; and a firstcoating comprising a carbon nanotube-infused fiber material in surfaceengagement with and at least partially covering the outer surface ofsaid heat absorbing element; whereby solar radiation incident onto saidfirst coating is received, absorbed, and converted to heat energy, andthe heat energy is transferred from said first coating to said heatabsorbing element.
 2. The solar receiver apparatus of claim 1, whereinsaid heat absorbing element has a first end and a second end, wherein aheat transfer fluid enters said heat absorbing element at said first endand exits from said heat absorbing element at said second end.
 3. Thesolar receiver of claim 1, wherein said heat absorbing element comprisesa heat pipe.
 4. The solar receiver of claim 1, wherein said heatabsorbing element comprises a metal.
 5. The solar receiver of claim 1,wherein said heat absorbing element has grooves to sized accommodatesaid CNT-infused fiber material.
 6. The solar receiver of claim 1,wherein said CNT-infused fiber material comprises a carbonnanotube-infused fiber tow.
 7. The solar receiver of claim 1, furthercomprising an environmental coating integrated within said first coatingto form a composite.
 8. The solar receiver of claim 7, wherein saidenvironmental coating comprises a ceramic matrix material.
 9. The solarreceiver of claim 7 further comprising metal particles.
 10. The solarreceiver of claim 1, further comprising an environmental coatingdisposed on said first coating, wherein said environmental coatingcomprises a low-emissivity coating.
 11. The solar receiver of claim 1,further comprising an environmental coating comprising a metal.
 12. Thesolar receiver of claim 1, further comprising an environmental coatingcomprising an anti-reflective material.
 13. The solar receiver of claim1, further comprising an annulus surrounding said first coating and saidheat absorbing element creating a gap.
 14. The solar receiver of claim13, wherein the gap comprises air.
 15. The solar receiver of claim 13,wherein the gap is evacuated.
 16. The solar receiver apparatus of claim1, wherein said apparatus is configured to integrate with a powergeneration system.
 17. A multilayer coating for a solar receiver devicecomprising: a first coating comprising a CNT-infused fiber material; andan environmental coating disposed on said first coating.
 18. The coatingof claim 17, wherein said first coating further comprises a ceramicmatrix.
 19. The coating of claim 17, wherein said first coating furthercomprises metal particles.
 20. The coating of claim 17, wherein saidenvironmental coating comprises a metal film.
 21. The coating of claim17, wherein said environmental coating comprises an anti-reflectivecoating.
 22. The coating of claim 17, wherein said environmental coatingcomprises a low emissivity coating.